11,984 research outputs found
Magnetic field effects in few-level quantum dots: theory, and application to experiment
We examine several effects of an applied magnetic field on Anderson-type
models for both single- and two-level quantum dots, and make direct comparison
between numerical renormalization group (NRG) calculations and recent
conductance measurements. On the theoretical side the focus is on
magnetization, single-particle dynamics and zero-bias conductance, with
emphasis on the universality arising in strongly correlated regimes; including
a method to obtain the scaling behavior of field-induced Kondo resonance shifts
over a very wide field range. NRG is also used to interpret recent experiments
on spin-1/2 and spin-1 quantum dots in a magnetic field, which we argue do not
wholly probe universal regimes of behavior; and the calculations are shown to
yield good qualitative agreement with essentially all features seen in
experiment. The results capture in particular the observed field-dependence of
the Kondo conductance peak in a spin-1/2 dot, with quantitative deviations from
experiment occurring at fields in excess of 5 T, indicating the eventual
inadequacy of using the equilibrium single-particle spectrum to calculate the
conductance at finite bias.Comment: 15 pages, 12 figures. Version as published in PR
Correlated electron physics in multilevel quantum dots: phase transitions, transport, and experiment
We study correlated two-level quantum dots, coupled in effective 1-channel
fashion to metallic leads; with electron interactions including on-level and
inter-level Coulomb repulsions, as well as the inter-orbital Hund's rule
exchange favoring the spin-1 state in the relevant sector of the free dot. For
arbitrary dot occupancy, the underlying phases, quantum phase transitions
(QPTs), thermodynamics, single-particle dynamics and electronic transport
properties are considered; and direct comparison is made to conductance
experiments on lateral quantum dots. Two distinct phases arise generically, one
characterised by a normal Fermi liquid fixed point (FP), the other by an
underscreened (USC) spin-1 FP. Associated QPTs, which occur in general in a
mixed valent regime of non-integral dot charge, are found to consist of
continuous lines of Kosterlitz-Thouless transitions, separated by first order
level-crossing transitions at high symmetry points. A `Friedel-Luttinger sum
rule' is derived and, together with a deduced generalization of Luttinger's
theorem to the USC phase (a singular Fermi liquid), is used to obtain a general
result for the T=0 zero-bias conductance, expressed solely in terms of the dot
occupancy and applicable to both phases. Relatedly, dynamical signatures of the
QPT show two broad classes of behavior, corresponding to the collapse of either
a Kondo resonance, or antiresonance, as the transition is approached from the
Fermi liquid phase; the latter behavior being apparent in experimental
differential conductance maps. The problem is studied using the numerical
renormalization group method, combined with analytical arguments.Comment: 22 pages, 18 figures, submitted for publicatio
Seawater transfer alters the intestinal microbiota profiles of Atlantic salmon (Salmo salar L.)
This study was funded by a BBSRC Eastbio PhD studentship to CED and BBSRC grant BB/M026604/1. The authors wish to thank Ana Rita Sancho Silva for facilitating the sampling for the experiment. Furthermore we would like to express our gratitude to Ian and Alastair Fraser for their support at the SFF fish farms on the Isle of Mull.Peer reviewedPublisher PD
Helicity operators for mesons in flight on the lattice
Motivated by the desire to construct meson-meson operators of definite
relative momentum in order to study resonances in lattice QCD, we present a set
of single-meson interpolating fields at non-zero momentum that respect the
reduced symmetry of a cubic lattice in a finite cubic volume. These operators
follow from the subduction of operators of definite helicity into irreducible
representations of the appropriate little groups. We show their effectiveness
in explicit computations where we find that the spectrum of states interpolated
by these operators is close to diagonal in helicity, admitting a description in
terms of single-meson states of identified J^{PC}. The variationally determined
optimal superpositions of the operators for each state give rapid relaxation in
Euclidean time to that state, ideal for the construction of meson-meson
operators and for the evaluation of matrix elements at finite momentum.Comment: 25 pages, 14 figures; v2: minor changes to reflect journal versio
The Role Body-Esteem Plays in Impairment Associated with Hair-Pulling and Skin Picking in Adolescents
Trichotillomania (hair pulling disorder, HPD) and pathological skin picking (PSP) are associated with significant rates of psychosocial impairment and distress. Little research has addressed the physical consequences and associated impairment in youth (e.g., poor body-esteem). The present study explores the relationship between body-esteem, skin picking (SP), and pulling-related impairment in a sample of adolescents with primary HPD. Ninety four adolescents who pull their hair, 40 of whom also pick their skin, were recruited via internet-sampling as part of the Child and Adolescent Trichotillomania Impact Study (CA-TIP). All youth and a parent completed anonymous questionnaires online assessing psychiatric symptoms, repetitive behaviors, and psychosocial impairment, among other variables. Appearance-based body-esteem was not found to be predictive of more severe psychosocial impairment in these youth. However, SP, in combination with HPD, contributed to worse appearance-based body-esteem above and beyond symptoms of HPD alone. The current study suggests that psychosocial functioning in youth with HPD is less impacted by body-esteem or pulling than other factors (e.g., depression and anxiety), and that SP contributes to lowered body-esteem. These findings suggest the importance of addressing body-esteem in case conceptualization for youth with both HPD and SP. Further research is required to confirm these suggestions
Millimeter and Submillimeter Survey of the R Corona Australis Region
Using a combination of data from the Antarctic Submillimeter Telescope and
Remote Observatory (AST/RO), the Arizona Radio Observatory Kitt Peak 12m
telescope and the Arizona Radio Observatory 10m Heinrich Hertz Telescope, we
have studied the most active part of the R CrA molecular cloud in multiple
transitions of Carbon Monoxide, HCO and 870\micron continuum emission.
Since R CrA is nearby (130 pc), we are able to obtain physical spatial
resolution as high as 0.01pc over an area of 0.16 pc, with velocity
resolution finer than 1 km/s. Mass estimates of the protostar driving the
mm-wave emission derived from HCO, dust continuum emission and kinematic
techniques point to a young, deeply embedded protostar of 0.5-0.75
M, with a gaseous envelope of similar mass. A molecular outflow is
driven by this source that also contains at least 0.8 M of molecular
gas with 0.5 L of mechanical luminosity. HCO lines show the
kinematic signature of infall motions as well as bulk rotation. The source is
most likely a Class 0 protostellar object not yet visible at near-IR
wavelengths. With the combination of spatial and spectral resolution in our
data set, we are able to disentangle the effects of infall, rotation and
outflow towards this young object.Comment: 29 pages, 9 figures. Accepted for publication in the Astrophysical
Journa
Confinement of the Sun's interior magnetic field: some exact boundary-layer solutions
High-latitude laminar confinement of the Sun's interior magnetic field is
shown to be possible, as originally proposed by Gough and McIntyre (1998) but
contrary to a recent claim by Brun and Zahn (A&A 2006). Mean downwelling as
weak as 2x10^-6cm/s -- gyroscopically pumped by turbulent stresses in the
overlying convection zone and/or tachocline -- can hold the field in
advective-diffusive balance within a confinement layer of thickness scale ~
1.5Mm ~ 0.002 x (solar radius) while transmitting a retrograde torque to the
Ferraro-constrained interior. The confinement layer sits at the base of the
high-latitude tachocline, near the top of the radiative envelope and just above
the `tachopause' marking the top of the helium settling layer. A family of
exact, laminar, frictionless, axisymmetric confinement-layer solutions is
obtained for uniform downwelling in the limit of strong rotation and
stratification. A scale analysis shows that the flow is dynamically stable and
the assumption of laminar flow realistic. The solution remains valid for
downwelling values of the order of 10^-5cm/s but not much larger. This suggests
that the confinement layer may be unable to accept a much larger mass
throughput. Such a restriction would imply an upper limit on possible internal
field strengths, perhaps of the order of hundreds of gauss, and would have
implications also for ventilation and lithium burning.
The solutions have interesting chirality properties not mentioned in the
paper owing to space restrictions, but described at
http://www.atmos-dynamics.damtp.cam.ac.uk/people/mem/papers/SQBO/solarfigure.htmlComment: 6 pages, 3 figures, to appear in conference proceedings: Unsolved
Problems in Stellar Physic
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